Operation of diaphragm monopolar electrolyzers for chlor-alkali electrolysis is improved by providing at least part of the anodes in their upper portion with hydrodynamic baffles (s) capable of generating a plurality of lifting and downcoming recirculation motions of the mixed anolyte-gas phase and of the anolyte separated from gas, respectively, which baffles are characterized by their superior edge or overflow holes located under the free surface of the anolyte, resulting in a reduction of the cell voltage and an increase in the faradic efficiency and the quality of the products.